The Plant Viruses Volume 2 THE ROD-SHAPED PLANT VIRUSES

THE VIRUSES

Series Editors HEINZ FRAENKEL-CONRAT, University of California

Berkeley, California

ROBERT R. WAGNER, University of Virginia School of Medicine Charlottesville, Virginia

THE VIRUSES: Catalogue, Characterization, and Classification Heinz Fraenkel-Conrat

THE ADENOVIRUSES Edited by Harold S. Ginsberg

THE HERPESVIRUSES, Volumes 1-3 • Edited by Bernard Roizman Volume 4 • Edited by Bernard Roizman and Carlos Lopez

THE PAPOVAVIRIDAE Volume 1 • Edited by Norman P. Salzman

THE PARVOVIRUSES Edited by Kenneth I. Berns

THE PLANT VIRUSES Volume 1 • Edited by R. I. B. Francki Volume 2. Edited by M. H. V. Van Regenmortel and Heinz Fraenkel-Conrat

THE REOVIRIDAE Edited by Wolfgang K. Joklik

THE TOGAVIRIDAE AND FLA VIVIRIDAE Edited by Sondra Schlesinger and Milton J. Schlesinger

The Plant Viruses Volume 2 THE ROD-SHAPED PLANT VIRUSES

Edited by

M. H. V. VAN REGENMORTEL Institute of Molecular and Cellular Biology Strasbourg, France

and

HEINZ FRAENKEL-CONRA T Department of Moleculo.r Biology and Virus Laboratory University of California Berkeley, California

PLENUM PRESS • NEW YORK AND LONDON

Library of Congress Cataloging in Publication Data

The Plant viruses.

(Viruses) Contents: v. 2. The rod-shaped plant viruses/edited by M. H. V. Van Regenmortel

and Heinz Fraenkel-Conrat. Bibliography: v. 2, p. Includes index. 1. Plant viruses. 2. Tobacco mosaic virus. I. Van Regenmortel, M. H. V. II.

Fraenkel-Conrat, Heinz, 1910- III. Series. QR402.P57 1986 576'.6483 86-9517

ISBN 978-1-4684-7028-4 ISBN 978-1-4684-7026-0 (eBook) DOI 10.1007/978-1-4684-7026-0

© 1986 Plenum Press, New York Softcover reprint of the hardcover 1st edition 1986 A Division of Plenum Publishing Corporation 233 Spring Street, New York, N.Y. 10013

All rights reserved

No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission from the Publisher

Contributors

J. G. Atabekov, Department of Virology, Moscow State University, Mos­cow 119899, USSR

A. C. Bloomer, MRC Laboratory of Molecular Biology, Cambridge CB2 2QH, England

Alan A. Brunt, Glasshouse Crops Research Institute, Littlehampton, West Sussex BN17 6LP, England

P. J. G. Butler, MRC Laboratory of Molecular Biology, Cambridge CB2 2QH, England

T. W. Carroll, Department of Plant Pathology, Montana State University, Bozeman, Montana 59717

R. G. Christie, Plant Virus Laboratory, Agronomy Department, Univer­sity of Florida, Gainesville, Florida 32611

V. V. Dolja, Department of Virology, Moscow State University, Moscow 119899, USSR

J. R. Edwardson, Plant Virus Laboratory, Agronomy Department, Uni­versity of Florida, Gainesville, Florida 32611

H. Fraenkel-Conrat, Virus Laboratory and Depaitment of Molecular Bi­ology, University of California, Berkeley, California 94720

Adrian Gibbs, Research School of Biological Sciences, Australian Na­tional University, Canberra, Australian Capital Territory, 2601, Australia

G. V. Gooding, Jr., Department of Plant Pathology, North Carolina State University, Raleigh, North Carolina 27695

B. D. Harrison, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, Scotland

Yoshimi Okada, Department of Biophysics and Biochemistry, Faculty of Science, University of Tokyo, Hongo Bunkyo-ku, Tokyo, 113, Japan

Peter Palukaitis, Department of Plant Pathology, Cornell University, Ith­aca, New York 14853

D. J. Robinson, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, Scotland

v

vi CONTRIBUTORS

Satyabrata Sarkar, Institute of Plant Medicine, University of Hohenheim, 7000 Stuttgart 70, West Germany

Eishiro Shikata, Department of Botany, Faculty of Agriculture, Hokkaido University, Sapporo, Japan

M. H. V. Van Regenmortel, Institut de Biologie Moleculaire et Cellulaire du CNRS, 67084 Strasbourg Cedex, France

Anupam Varma, Division of Mycology and Plant Pathology, Indian Ag­ricultural Research Institute, New Delhi 110012, India

Carl Wetter, Department of Botany, University of Saarland, D-6600 Saar­briicken, West Germany

Milton Zaitlin, Department of Plant Pathology, Cornell University, Ith­aca, New York 14853

F. W. Zettler, Plant Pathology Department, University of Florida, Gaines­ville, Florida 32611

Preface

This volume of the series The Plant Viruses is devoted to viruses with rod-shaped particles belonging to the following four groups: the toba­moviruses (named after tobacco mosaic virus), the tobraviruses (after to­bacco rattle), the hordeiviruses (after the latin hordeum in honor of the type member barley stripe mosaic virus), and the not yet officially rec­ognized furoviruses (fungus-transmitted rod-shaped viruses, Shirako and Brakke, 1984).

At present these clusters of plant viruses are called groups instead of genera or families as is customary in other areas of virology. This pe­culiarity of plant viral taxonomy (Matthews, 1982) is due to the fact that the current Plant Virus Subcommittee of the International Committee of Taxonomy of Viruses is deeply split on what to call the categories or ranks used in virus classification.

Some plant virologists believe that the species concept cannot be applied to viruses because this concept, according to them, necessarily involves sexual reproduction and genetic isolation (Milne, 1984; Murant, 1985). This belief no doubt stems from the fact that these authors restrict the use of the term species to biological species. According to them, a collection of similar viral isolates and strains does constitute an individ­ual virus, i.e., it is a taxonomy entity separate from other individual viruses. However, instead of calling these elementary units of classifica­tion: different virus species, these authors call them viruses, refusing to acknowledge the fact that there is a need for a word to signify that one is not referring to a viral object but to a taxonomic construct (an abstract concept as opposed to a collection of material objects).

Some of the protagonists of this viewpoint (Milne, 1984) go as far as to deny that there is a difference between the existence of a concept and the existence of material objects, and they reject the view that clas­sifications are conceptual constructions. It is not clear why the word species should be used only in the sense of biological species defined by gene pools and breeding barriers, and why a phenetic or morphological

vii

viii PREFACE

definition of species, as practiced by numerical taxonomists, should not prove acceptable for classifying viruses.

The material objects, i.e., the viruses discussed in the present vol­ume, have been grouped according to the divisions embodied in the CMII AAB descriptions of plant viruses. In the past, most tobamoviruses such as ribgrass mosaic virus or cucumber virus 4 were considered strains of tobacco mosaic virus (TMV).

As new CMII AAB descriptions of tobamoviruses are published and the corresponding taxonomic entity is given a name, it has become stan­dard practice to consider that these names now stand for individual vi­ruses and not for individual strains of TMV. According to standard tax­onomic parlance, these individual viruses have thus become de facto species (Matthews, 1985). In addition to the species TMV, six other spe­.;ies of tobamoviruses are discussed at length in the present volume (Chapters 9-14). Following standard taxonomic categories and terminol­ogy, the tobamovirus group can be considered a genus made up of a ser­ologically homogeneous cluster of related species. Similarly the furo-, tobra-, and hordeivirus groups can be taken to represent three separate plant virus genera.

About a third of this volume is devoted to TMV, a deliberate choice in view of the importance of this virus for the development of virology. Although the common TMV strain is perhaps the best studied object in the whole of virology, the reader will discover that the boundaries of our knowledge concerning this virus are still being extended and that the end is not yet in sight. The open-ended nature of scientific knowledge is never more apparent than in the description of a subject that has been very extensively researched.

Strasbourg Berkeley

REFERENCES

M. H. V. Van Regenmortel H. Fraenkel-Conrat

Milne, R. G., 1984, The species problem in plant virology, Microbio1. Sci. 1:113-117. Matthews, R. E. F., 1982, Classification and nomenclature of viruses, fourth report of the

international committee on taxonomy of viruses, Intervirology 17:1-199. Matthews, R. E. F., 1985, Viral taxonomy, Microbio1. Sci. 2:74-76. Murant, A. F., 1985, Taxonomy and nomenclature of viruses, Microbio1. Sci. 2:218-220. Shirako, Y., and Brakke, M. K., 1984, Two purified RNAs of soil-borne wheat mosaic virus

are needed for infection, f. Gen. Viro1. 65:119-127.

Contents

I. Tobamoviruses

Chapter 1

Tobacco Mosaic Virus: The History of Tobacco Mosaic Virus and the Evolution of Molecular Biology ............................................... 5

H. Fraenkel-Conrat

Chapter 2

Tobacco Mosaic Virus: Structure and Self-Assembly

A. C. Bloomer and P. r. G. Butler

I. Introduction .......................................................................... 19 II. Structure ............................................................................... 20

A. Polymorphic Aggregates of Protein .................................. 21 B. Detailed Molecular Structures ......................................... 25

III. Self-Assembly ........................................................................ 34 A. Nucleation ....................................................................... 34 B. Elongation ....................................................................... 40

IV. Conclusions .......................................................................... 51 References ...................................................................................... 52

Chapter 3

Tobacco Mosaic Virus: Mutants and Strains

Satyabrata Sarkar

I. Introduction .......................................................................... 59 II. Isolation of Variants .............................................................. 60

ix

x CONTENTS

A. Source and Properties of a Few Strains and Mutants of TMV ............................................................................ 60

B. Induced Mutants .............................................................. 63 C. Apparently Induced but Possibly Spontaneous

Mutants ........................................................................... 68 III. Contribution of Mutants and Strains of TMV to Virology

and Molecular Biology ........................................................... 69 References ...................................................................................... 72

Chapter 4

Tobacco Mosaic Virus: Antigenic Structure

M. H. V. Van Regenmortel

I. Introduction .......................................................................... 79 II. Methods Used to Study TMV Antigenicity......... .................. 80

A. Fragmentation of the Protein ........................................... 81 B. Studies with Synthetic Peptides ...................................... 82 C. Cross-Reactivity Studies with Antipeptide Antibodies .... 83 D. Cross-Reactivity Studies with TMV Mutants .................. 84 E. Studies with Monoclonal Antibodies ............................... 85

III. Types of Epitopes Identified in TMV and TMVP ........ ..... ...... 86 A. Continuous Epitopes ....................................................... 86 B. Discontinous Epitopes ..................................................... 90 C. Neotopes ......................................................................... 93 D. Cryptotopes ..................................................................... 94

IV. Determination of the Binding Constant of TMV Antibodies ................................................................ 96

V. Applications of Serology in TMV Studies ........ ...................... 98 A. Virus Detection ............................................................... 98 B. Virus Classification ......................................................... 99

References ...................................................................................... 100

Chapter 5

Tobacco Mosaic Virus: Infectivity and Replication

Peter Palukaitis and Milton Zaitlin

I. Introduction .......................................................................... 105 II. Infection and Pathogenicity ................................................... 106

A. Viral Ingress into the Plant .............................................. 106 B. Site of Viral Entry into the Plant and the Cell ................ 107 C. Virus Movement from Cell to Cell .................................. 108 D. Long-Distance Virus Movement ...................................... 109 E. Symptom Determinants .............. ........ .... ...... .................. 110

III. Replication ............................................................................ 111 A. Introduction ..................................................................... III

CONTENTS xi

B. Gene Organization ........................................................... III C. TMV Life Cycle .................... .................................... ....... 119 D. Cross-Protection versus Replication ................................ 124

References ...................................................................................... 126

Chapter 6

Tobacco Mosaic Virus: Epidemiology and Control

G. V. Gooding, Tr.

I. Introduction .......................................................................... 133 II. The Disease ........................................................................... 133

A. Etiology ........................................................................... 133 B. Symptoms ............................... ......................................... 135 C. Identification ........................... ........................................ 135 D. Effects on the Host .......................................................... 135 E. Economic Importance ...................................................... 137

III. Epidemiology ......................................................................... 137 A. Sources of Inoculum ........................................................ 137 B. Transmission ............................ ....................................... 139 C. Environmental Factors ..................................................... 140

IV. Control .................................................................................. 141 A. Resistance ........................................................................ 141 B. Tolerance ......................................................................... 142 C. Prevention of Primary Infection ...................................... 143 D. Prevention of Secondary Spread ....................................... 145 E. Chemical ......................................................................... 146 F. Cross-Protection and Interference .................................... 147

V. Discussion ........ .......... ........................................................... 147 References ...................................................................................... 148

Chapter 7

Tobacco Mosaic Virus: Cytopathological Effects

T. R. Edwardson and R. G. Christie

I. Introduction .......................................................................... 153 II. Crystalline Inclusions ........................................................... 154

III. Paracrystals .................. .................... .......................... ........... 156 IV. Angled-Layer Aggregates ....................................................... 157 V. X-Bodies ................................................................................ 157

VI. Virus Aggregates in Chloroplasts .......................................... 158 VII. Other Types of Cytoplasmic Inclusions ................................ 159

VIII. Nuclear Inclusions ................................................................ 161 IX. Concluding Remarks ............................................................. 162

References ...................................................................................... 163

xii CONTENTS

Chapter 8

Tobamovirus Classification

Adrian Gibbs

I. Relationship among the Definitive Tobamoviruses ............... 168 A. Coat Protein .................................................................... 168 B. 1/30K" Protein .................................................................. 174 C. 3' Noncoding Region of the Genome ............................... 174 D. The Genome .................................................................... 174 E. Host Range ...................................................................... 176

II. Relationships between Tobamoviruses and Other Viruses .... 177 III. Conclusions .......................................................................... 178

References ...................................................................................... 178

Chapter 9

Tomato Mosaic Virus

Alan A. Brunt

I. Introduction .......................................................................... 181 II. The Virus and Its Strains ....................................................... 181

A. Symptomatologically Distinct Strains ............................. 183 B. Strains (or Pathotypes) Differentiated by Host

Resistance Genes ............................................................. 186 III. Properties and Composition .................................................. 186

A. Properties ........................................................................ 186 B. Composition .................................................................... 187

IV. Affinities ............................................................................... 192 A. Serological Relationships ..................... .... ........................ 192 B. Molecular Hybridization Analyses ................................... 193 C. Amino Acid Analyses ...................................................... 194

V. Intracellular Occurrence ........................................................ 194 VI. Epidemiology and Control ..................................................... 195

A. Epidemiology ................................................................... 195 B. Control ............................................................................ 195

References ...................................................................................... 197

Chapter 10

Tobacco Mild Green Mosaic Virus

Carl Wetter

I. Introduction .......................................................................... 205 II. Biological Properties .............................................................. 206

A. Virus Strains and Their Economic Importance ................ 206

CONTENTS xiii

B. Host Range and Symptomatology .................................... 206 C. Cytopathological Effects .................................................. 207

m. Physicochemical Properties ................................................... 210 A. Purification ...................................................................... 210 B. Particle Morphology ........................................................ 210 C. Virion Properties .............................................................. 211 D. Nucleotide Sequence Homology ...................................... 211 E. Protein Composition ....................................................... 211

IV. Serology ................................................................................. 212 V. Interaction of TMGMV with Other Tobamoviruses .............. 213

A. Cross-Protection and Interference .................................... 213 B. Interference at an Early Stage of Infection ....................... 215 C. Structural Interactions of Particles .................................. 216

VI. Virus Mutation versus Host-Induced Mutation ..................... 216 References ...................................................................................... 217

Chapter 11

Ribgrass Mosaic Virus

Carl Wetter

I. Introduction .......................................................................... 221 II. Biological Properties .............................................................. 221

A. Isolates and Strains of RMV and Their Economic Importance ...................................................... 221

B. Symptomatology and Host Range ........ .................. .......... 222 C. Cytopathology ................................................................. 224

III. Physicochemical Properties ................................................... 225 A. Purification ...................................................................... 225 B. Particle Morphology ........................................................ 225 C. Virion Properties .............................................................. 227 D. The Coat Protein ............................................................. 227 E. Structural Interactions ..................................................... 228

IV. Serology ................................................................................. 228 References ...................................................................................... 230

Chapter 12

Odontoglossum Ringspot Virus

r R. Edwardson and F. W. Zettler

I. Historical Review .................................................................. 233 II. Structure ........ ............................ .......... ...................... ........... 233

III. ORSV Mutants or Strains ..................................... ....... .......... 235 IV. Antigenic Structure ............................................................... 236 V. Pathogenicity ........................................................................ 236

xiv CONTENTS

VI. Transmission, Epidemiology, Control .............. ..................... 238 VII. Cytopathology ....................................................................... 241

References ...................................................................................... 243

Chapter 13

Sunn-Hemp Mosaic Virus

Anupam Varma

I. Introduction .......................................................................... 249 II. Important Diseases and Their Geographical Distribution ..... 249

III. Host Range and SYlllptoms ................................................... 250 A. Diagnostic Species ........................................................... 250 B. Inclusion Bodies .............................................................. 251 C. Physiological Changes ..................................................... 251 D. Interaction with Viruses and Fungi ................................. 252

IV. Strains ................................................................................... 252 A. Probable Strains ............................................................... 252 B. Thermophilic Strain ........................................................ 253 C. Host-Induced Changes ..................................................... 253

V. Transmission ......................................................................... 254 A. Vector Transmission ........................................................ 255 B. Seed Transmission ........................................................... 255

VI. Stability in Sap .... .................. ................ .......................... ...... 256 VII. Serological Relationships ....................................................... 256

VIII. Properties of Particles ............................................................ 257 A. Structure and Sedimentation ........................................... 257 B. Ultraviolet Absorption ..................................................... 257

IX. Particle Composition ............................................................. 257 A. Protein '" .................... .... .. ...... ................ .......... .... .. .. ........ 257 B. Nucleic Acid .................................................................... 260

X. Biological Significance of the Two Types of Particles ........... 261 A. Relative Production of Short and Full-Length Particles

in Plants .......................................................................... 261 B. Separation of Short and Full-Length Particles .................. 261 C. Infectivity ........................................................................ 261 D. Translation of RNAs from Short and Full-Length

Particles in Cell-Free Systems ......................................... 261 E. Polyribosome-Associated Coat Protein mRNA

in Plants .......................................................................... 262 XI. Ecology and Control.......... .................................................... 263

XII. General Remarks ................................................................... 264 References .... .................................................................................. 264

CONTENTS

Chapter 14

Cucumber Green Mottle Mosaic Virus

Yoshimi Okada

xv

I. Introduction .......................................................................... 267 II. Strains ................................................................................... 268

A. Type Strain (Cucumber Virus 3) ...................................... 268 B. Cucumber Aucuba Mosaic Strain (Cucumber Virus 4) .... 268 C. Watermelon Strain ........................................................... 268 D. Japanese Cucumber Strain ............................................... 269

III. Virus Particle ........................................................................ 269 IV. Genomic and Subgenomic RNA ............................................ 269

A. Nucleotide Composition and Sequence of Genomic RNA ................................................................. 269

B. Coat Protein and 30K Protein Cistron ............................. 270 C. Nucleotide Sequence of the Assembly Origin and

Comparison with That of TMV ....................................... 271 D. The 3' Noncoding Region ................................................ 272 E. Subgenomic RNA ............................................................ 273

V. Coat Protein .......................................................................... 273 A. Amino Acid Sequence ..................................................... 273 B. Anomalous Mobility of the CGMMV Coat Protein on

SDS-Polyacrylamide Gel Electrophoresis ........................ 274 C. Aggregation State of the Coat Protein ............................. 274 D. RNA Location in the CGMMV Protein Disk .................. 275

VI. Virus Particle Assembly in Vitro ........................................... 277 A. Specificity of the Assembly Reaction in Vitro ................. 277 B. Studies on TMV Assembly Using the

CGMMV Protein ............................................. ,............... 277 References ...................................................................................... 278

Chapter 15

Miscellaneous Tobamoviruses

Alan A. Brunt

I. Introduction .......................................................................... 283 II. The Viruses and Their Natural Occurrence .......................... 283

A. The Viruses and Their Hosts ........................................... 283 B. Intracellular Occurence of the Viruses ............................ 292

III. Properties .............................................................................. 292 A. General Properties of Particles ........................................ 292 B. Composition .................................................................... 294

IV. Serological Affinities ............................................................. 296 References ............... .. .... ................................................................. 300

xvi CONTENTS

II. Fungus-Transmitted and Similar Labile Rod-Shaped Viruses

Chapter 16

Fungus-Transmitted and Similar Labile Rod-Shaped Viruses

Alan A. Brunt and Eishiro Shikata

I. Introduction .......................................................................... 305 II. The Viruses and Their Natural Hosts ................................... 309

A. The Viruses and the Diseases They Cause ...................... 309 B. Intracellular Occurrence and Cytopathology ................... 312

III. Transmission and Disease Control ........................................ 312 A. Fungal Vectors and Virus-Vector Relationships .............. 312 B. Seed Transmission .................................................. ......... 313

IV. Virus Purification .................................................................. 314 V. Properties of Virus Particles .................................................. 315

A. Morphology, Size, Structure, and Infectivity .................... 315 B. General Properties ........................................................... 322 C. Composition .................................................................... 324 D. Serological Affinities ....................................................... 327

VI. Taxonomy ............................................................................. 329 References ...................................................................................... 330

Chapter 17

Tobraviruses

III. Tobraviruses

B. D. Harrison and D. J. Robinson

I. Recognition and Characteristics of the Tobravirus Group ..... 339 A. Members and Their Geographical Distribution ............... 339 B. General Properties of Tobraviruses .................................. 339

II. Tobravirus Particles and Their Components ......................... 341 A. Particle Morphology and Structure .................................. 341 B. Particle Protein Size and Structure .................................. 343 C. The RNA Species and Their Structure ............................ 343 D. Particle Protein Aggregation and

Nucleoprotein Reassembly .............................................. 344 III. Variation and Relationships .................................................. 345

A. Biological Variation ......................................................... 345 B. Antigenic Variation ......................................................... 346 C. Cross-Protection between Isolates ........................ ........... 346 D. Pseudorecombinant Formation ........................................ 346

CONTENTS xvii

E. RNA Sequence Homologies ............................................. 347 F. Pseudorecombination in Nature ...................................... 348 G. Classification of Tobraviruses .......................................... 348

IV. Replication and Genome Strategy . ............ .................. ..... ..... 349 A. Multiplication in Plants .................................................. 349 B. Replication in Protoplasts ............ .... ................................ 349 C. Interactions with Cells .................. ........ .......................... 350 D. Strategy of Genome Expression ....................................... 351 E. Distribution of Genetic Determinants ... .......................... 352

V. Natural History ..................................................................... 353 A. Diseases Caused .............................................................. 353 B. Nematode Transmission .................................................. 356 C. Seed Transmission ........................................................... 356 D. Ecology ............................................................................ 358 E. Control ............................................................................ 359

VI. Detection and Identification .................................................. 360 VII. Affinities with Other Virus Groups ....................................... 361

VIII. Special Features ..................................................................... 363 References ...................................................................................... 364

IV. Hordeiviruses

Chapter 18

Hordeiviruses: Biology and Pathology

T. W. Carroll

I. Introduction .......................................................................... 373 II. Barley Stripe Mosaic Virus ............................ ........................ 373

A. Main Characteristics ............................... ........................ 373 B. Geographical Distribution ............................................... 374 C. Host Range and Symptomatology .................................... 375 D. Strains ............................................................................. 376 E. Cytological Effects on Host ............................................ 378 F. Physiological Effects on Host .......................................... 378 G. Transmission through Pollen and Seed ............................ 380 H. Methods of Disease Diagnosis and Virus Detection ........ 382 I. Economic Significance ..................................................... 384 J. Epidemiology................................................................... 385 K. Control ............................................................................ 386

III. Poa Semilatent Virus .................. .............. .................... ......... 388 IV. Lychnis Ringspot Virus ... ............ ...... ..... ... .... ............ ...... ...... 390 V. Closing Remarks ................................................................... 391

References ...................................................................................... 392

xviii CONTENTS

Chapter 19

Hordeiviruses: Structure and Replication

J. G. Atabekov and V. V. Dolia

I. Introduction .......................................................................... 397 II. General Biological Characteristics ......................................... 398

III. The Virions and Their Substructural Components ................ 399 A. Purified BSMV ................................................................. 399 B. Coat Protein .................................................................... 400 C. Polymerization of the Capsid Protein and

Virion Reassembly ........................................................... 401 IV. Genome Structure and Expression ........................................ 404

A. Multipartite Nature of the Genome ................................ 404 B. Sequence Relationship among BSMV RNAs .................... 406 C. Changes in the Set of Virion RNAs upon Laboratory

Passaging of BSMV .......................................................... 406 D. Structure of the 3'-Terminal Region of BSMV RNAs ...... 407 E. Translation in Vitro of Individual BSMV RNAs .............. 409 F. Subgenomic RNAs of BSMV ............................................ 413 G. Replication of BSMV ....................................................... 414 H. BSMV-Induced Mutagenesis in Host Plants ..................... 414

V. Concluding Remarks ............................................................. 415 References ...................................................................................... 417

Index .............................................................................................. 421